1. Field of the Invention
This invention relates to laminated transparent assemblies suitable for use as windscreen for high speed vehicles such as aircraft or railway locomotives.
2. Description of the Prior Art
A known form of aircraft windscreen comprises the following components:
A thick inner sheet of highly toughened glass which carries major loads due to internal pressurisation of the aircraft whilst in flight.
A thinner outer sheet of highly or lightly toughened glass which is usually provided with a transparent elecro-conductive film on its inner surface, power being applied to the film to provide heat primarily for de-icing and demisting of the surface of the windscreen when in service.
A thick interlayer of a plastics material such as polyvinylbutyral bonding the inner and outer sheets of glass together.
In another known form of aircraft windscreen two thick sheets of highly toughened glass are laminated together with a thick interlayer of plastics material therebetween.
In the kind of aircraft windscreen which includes these two forms and which in more general terms comprises at least two sheets of glass bonded together by a thick plastics interlayer, the interlayer has been adapted to provide for edge location of the windscreen in the aircraft structure. In one such case the edge of the interlayer projects beyond the periphery of the sheets of glass and a metal strip, for example of aluminum or stainless steel, is bonded in the projecting marginal portion of the interlayer in a plane substantially parallel to the surfaces of the windscreen. Mounting holes are drilled through the projecting edges of the interlayer and the metal strip, for bolting the windscreen to the edges of an opening in the aircraft structure.
The interlayer is generally heated to an optimum temperature in service, for example by the medium of the electroconductive film mentioned above. The interlayer plays two major structural roles.
If the windscreen is struck by a bird whilst the aircraft is in flight the dynamic loading can cause failure of the glass components of the windscreen. In this case the interlayer deforms inwardly and the kinetic energy of the bird is absorbed by deformation of the interlayer. This deformation leads to considerable loads at the located edge of the interlayer. The metal strip which is embedded in the edges of the interlayer reinforces the interlayer against the loads which arise.
Aircraft windscreens also have to be designed in such a manner that in the event of failure of the glass components when the aircraft is at high altitudes depressurisation of the aircraft is prevented. The provision of an interlayer reinforced and bolted at its edges to the aircraft structure gives such protection. With the glass components failed and with the interlayer at its optimum temperature, the interlayer can deform outwards into a "ballon" shape thus resisting the internal pressurisation for long enough for the aircraft to descend to an altitude at which internal pressurisation is no longer necessary.
However the use of a reinforcing metal insert embedded in the edges of the interlayer of the windscreen as described above is known to have a deleterious effect on windscreen reliability. The materials used in the windscreen construction have different physical properties relative to each other and relative to the material of the surrounding aircraft structure. Differences between the thermal expansion co-efficients of the materials involved in a particularly relevant factor. In the range of environmental temperatures experienced in aircraft usage, the glass components of the windscreen will expand and contract thermally relative one to another and relative to the aircraft structure. In particular, because the edges of the interlayer in the windscreen are rigidly located with respect to the aircraft structure by the metal insert, thermal expansion and contraction of the interlayer relative to the aircraft structure can give rise to stresses at the edges of the interlayer. Such stresses are also contributed to by thermal expansion and contraction of the glass components of the windscreen relative to the interlayer and by mechanical deformation of the windscreen and the aircraft structure under the loads experienced in flight. Such thermally and mechanically induced stresses can cause the phenomena known as "delamination" and "cold chipping". The stresses give rise to shear forces at the interface between the interlayer and the inner surface of the glass sheets around the edges of the windscreen. "Delamination" occurs when the shear forces which arise are sufficiently high to cause failure of the adhesive bond between the glass surfaces and the interlayer at the edges of the windscreen. On the other hand if the adhesive bond between the glass surfaces and the interlayer is sufficiently strong to resist delamination, failure can occur by chipping of the edges of the glass sheets by tensile stressing.
The risk of delamination and/or cold chipping is enhanced if the edges of the heated windscreen are allowed to become very cold when the aircraft is flying at high altitude, in very low ambient temperatures. The presence of a metal insert in the marginal portion of the interlayer of the windscreen, which insert has a thermal conductivity much higher than that of the interlayer, serves to extract heat from the edges of the windscreen. This results in the setting up of a temperature gradient across the edges of the windscreen giving rise to further differential thermal stress between the glass components and the interlayer of the windscreen which increases the likelihood of delamination or cold chipping at the edges of the windscreen.